Photoconductive layer separated from reactive opaque pattern by transparent conductive layer



Dec. 24, 1968 s. A. BYNUM 3,418,508

PHOTOCONDUCTIVE LAYER SEPARATED FROM REACTIVE OPAQUE PATTERN BYTRANSPARENT CQNDUCTIVE LAYER Filed Aug. '23, 1967 STANLEY A B YNUMINVENTOR.

ATTORNEY United States Patent 0 M 3,418,508 PHOTOCONDUCTIVE LAYERSEPARATED FROM REACTIVE OPAQUE PATTERN BY TRANSPAR- ENT CONDUCTIVE LAYERStanley A. Bynum, Dallas, Tex., assignor to General 5 ElectrodynamicsCorporation, Garland, Tex., a corporation of Texas Continuation-impartof application Ser. No. 459,778, May 28, 1965. This application Aug. 23,1967, Ser. No. 662,715

10 Claims. (Cl. 313-65) 10 ABSTRACT OF THE DISCLOSURE CROSS REFERENCE TORELATED APPLICATION This application is a continuation-in-part ofapplication Ser. No. 459,778 filed May 28, 1965, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to devices which transduce optical or other radiated images intoelectrical signals, and more particularly it relates to photoconductivetargets of the type that are used in television camera pickup tubes, andto means for causing permanent black image patterns thereon.

Description of the prior art Various electronic devices are known whichutilize photoconductive materials for the production of electricalsignals from optical images or from images produced -by other types ofradiation. Among these devices is the vidicon television camera tube,and the following description will be principally in terms of theapplication of the present invention to the vidicon tube, although, aswill become apparent, the invention has equal applicability to manyother devices utilizing photoconductive materials. As is well known, anddescribed, for example, in US. Patent No. 2,745,032 to Forgue et al., avidicon camera tube consists of an electron gun and a target assemblycontained in a glass envelope, usually about six inches long and oneinch in diameter. The electron gun may be of the conventional type usedin other types of television pickup tubes. The target assembly usuallycomprises a film of lighttransparent, electrically conductive materialon the faceplate of the envelope and a coating of photoconductivematerial deposited upon the electrically conductive film. The target andthe gun are so arranged within the envelope that the electron beam fromthe gun scans the photoconductive surface of the target.

The use of a dark current reference mask on image pickup devices is afairly common practice. Such a mask consists of an optically opaquecovering over a portion of the photosensitive area of the device. Thecovered portion is intended to be electrically representative of thedark condition of the photosensitive area. Thus, such a mask permitselectrical subtraction, using appropriate circuitry well known in theart, of the electrical signal which represents the no-input condition ofthe device, this condition being commonly known as the dark current.

Such a dark current reference mask is particularly 3,418,508 PatentedDec. 24, 1968 useful in eliminating the effect, on an image pickupdevice, of changes in temperature. The dark current of most image pickupdevices increases with increasing ambient temperature so that the totaloutput signal will vary with fluctuations in temperature. It will beapparent that such a condition is highly undesirable in many instances.Another cause of changes in dark current is a change in applied voltage.In image devices such as the vidicon and the image Orthicon increases inthe output signal can be achieved by voltage increases in the targetvoltage, in the case of the vidicon, and in the negative voltage of thephotocathode with respect to the collector, in the case of the imageOrthicon. Upon increasing the output signal a corresponding increase indark current will result so that the dark current will constitute asubstantial portion of the total output signal. This detrimentalcondition can be compensated very effectively by the use of a darkcurrent reference mask.

Another application in which a dark current reference mask is useful isshuttered slow scan television. In such applications, the image ispicked up during a brief exposure and stored for several seconds orminutes while being read out by an electron beam. During the timerequired to scan an entire image, the dark signal can increasesubstantially in the last elements to be scanned over those firstscanned. This introduces an apparent defeet in the image pickup device,called shading.

Another instance in which a permanent black image is desired in thepicture produced by a television camera tube is when a pattern ofreticles is used, for example, for the purpose of forming referencemarks which provide a scale of reference for measurement. When thetelevision camera tube is used in a camera for viewing an object havingunknown lateral dimensions, such as, for example, configurations on themoon or on various planets, it is highly desirable to provide means formeasuring such lateral dimensions. Thus, two optically black signalproducing marks may be placed on the faceplate of the camera tube at aknown distance apart, and this known distance being reproduced by thereceiver can then be used as a scale to determine dimensions of objectsviewed by the camera. Such black reticles have been produced in the pastby depositing an opaque material in a desired reticle pattern on thefaceplate and then depositing the photoconductor on top of the opaquematerial so that the opaque material will block out light from portionsof the photoconductive coating, thereby producing a permanent blackimage at the location of the reticle on the receiver. In vidicontelevision camera tubes, opaque materials in the desired black imagepattern have been applied on the outside of the transparent windowthrough which the optical image is received so that a desired portion ofthe photoconductive area is shadowed from the image. This method has aserious drawback in that the opaque material is separated from thephotoconductive material by at least the thickness of the transparentwindow through which the image is admitted. This results in an opticalpenumbra on the photoconductive material in the region near the edge ofthe black reference mask or other image pattern. Such a partiallyshadowed region is rendered useless because it represents neitheroptical black nor full illumination, and in the case of a very thinreticle mark may cause a lightening of the black image so that thereticle is very difficult to see on the receiver.

Depositing the opaque material on the interior of the transparent windowor support so that it is in direct contact with the photoconductivecoating has also been found to be unsatisfactory because a permanentelectrical indication of optical blackness is not achieved. It has nowbeen found that the direct contact between the opaque material and thephotoconductive material is often unsatisfactory because there is achemical reaction between the opaque materials used and the usualphotoconductors such as selenium and antimony trisulfide. This causes adegradation of the photoconductive materials. This degradation manifestsitself as either an increase or a decrease in dark current in the areaaffected, rendering the area useless for establishing a reference levelof dark current or for dependable formation of a black reticle or otherpermanent black image.

SUMMARY OF THE INVENTION According to the present invention thephotoconductive material of the vidicon camera tube or the like isprotected from the deteriorating effect of the opaque material, andoptical penumbras are avoided, by applying the opaque material in thedesired pattern on the face-plate, coating the entire faceplate with aconductive coating which has no deteriorating effect on thephotoconductive material, and lastly depositing the photoconductivematerial on the conductive layer so that the photoconductor is fullyinsulated from the opaque material which reacts with it. The conductivelayer normally used is very thin, usually only a few microns thick, sothat the opaque markings are close enough to the photoconductive layerthat an optical penumbra problem is avoided.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 shows a longitudinal andpartly sectional view of one form of vidicon camera tube embodying thepresent invention; and

FIGURE 2 shows a faceplate of a vidicon camera tube embodying thepresent invention, with parts of the coatings broken away.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGURE 1 shows a vidicon typecamera tube, indicated generally :by the reference numeral 10, whichcomprises an evacuated envelope 12 having an electron gun 16 in one endthereof. The electron gun 16 may be any of the known types of electronguns and produces an electron beam directed toward the target electrode18 in the other end of the envelope 12. The electron beam is focused andscanned over the exposed surface of the v I target electrode by anyconventional means (not shown).

The target electrode 18 is attached to a metal ring 19 made of a metalsuch as Kovar which is sealed, by means well known in the art, to theedge of the target electrode and to the end of the envelope 12. Thetarget electrode comprises a transparent faceplate or substrate 20preferably made of glass or fused quartz or the like.

According to the present invention, the transparent faceplate hasapplied directly to its surface an optical mask 22 which comprises acoating of an opaque material around the edges of the circular faceplateand leaving a square or rectangular transparent opening 23. Of coursethe opaque material may be deposited in any desired configuration so asto form any desired shape of optically black image on thephotoconductive layer, such as, for example, a reticle pattern. Applieddirectly on top of the mask 22 and covering the opening as well as themask is a transparent conductive layer 24. Lastly, a photoconductivelayer 26 is applied directly on top of the conductive layer 24.

The various layers 22, 24, and 26 may be deposited by any convenientmeans well known in the art, such as, for example, vacuum evaporation,vapor reaction, plating, settling, spraying, or any other method whichproduces the desired physical and electrical characteristics.

The preferred opaque materials for use in the practice of the inventionare platinum and chromium, and alloys of each, although rhodium,palladium and their alloys also give good results. The metal isdeposited in a thickness suflicient to substantially preventtransmission therethrough of the particular radiation which it isdesired to mask. A preferred manner of accomplishing evaporation in thedesired pattern is to apply a contact evaporation mask directly over theportion which is not to be coated by the opaque material. Then theevaporation of the masking material is carried out, following which theevaporation mask is removed, leaving, in the case of a dark currentreference mask, the clear window as shown in FIGURE 2. Preferably theevaporation of the opaque material is continued until the opaquematerial has a thickness sufiicient to reduce transmission of radiationto no more than about /2%.

Various transparent conductive layers well known in the art which do notcause degradation of the photoconductors with which they are used may beused for the layer 24 herein. For example, tin oxide and indiumsesquioxide are chemically and electrically non-reactive withphotoconductors such as antimony trisulfide and selenium. Thephotoconductive layer is then applied on top of the transparentconductive layer. Thus, photoconductors such as antimony trisulfide andselenium, which would normally be degraded by interaction with suchopaque materials as platinum, chromium, rhodium, palladium and alloys ofeach, are fully protected from such interaction.

To utilize the mask layer 22 as an optically black reference mask in avidicon, the scanning pattern 30 of the electron beam, as somewhatdiagrammatically illustrated in FIGURE 2, should overlap the opaquematerial a small amount. Thus, the signal generated when the electronbeam is impinging on the opaque material may be used as a reference toindicate substantially complete absence of the radiation being viewed,and suitable electronic subtraction equipment (not shown) may comparethe signal with the signals generated during the remainder of the scanso that these signals Will be more truly indicative of the image beingscanned. If the electron beam scans a portion of the masked area on eachtraverse of the opening or window 23, then the reference signal levelwill be adjusted on each traverse to compensate for any changedconditions which may be encountered.

I do not wish to be bound by a particular theory as to the reason forthe problem solved by the present invention. However, it is believedthat many of the opaque materials normally used to block out light fromareas of a photoconductor undergo spontaneous chemical reaction with thephotoconductors, thereby degrading the performance of thephotoconductors. Furthermore, it is believed that transparent conductivecoatings such as tln oxide and indium sesquioxide do not reactspontaneously with photoconductors, so that the transparent conductivelayer will form a protective overlay for the opaque material and preventreaction with accompanying degradation of the photoconductive layer.This theory appears to be confirmed by analysis of various possiblecombinations of materials in the light of the principle of thermodynamicwhich states that a chemical reaction will proceed spontaneously in thedirection of the smallest (or greatest negative) total heat content(enthalpy) for all reactants and for all reaction products respectively.This total heat content can be computed for each direction of a chemicalreaction from the heats of formation of the compounds formed in thereaction, multiplied by the number of moles of each compound. Forexample, in the reaction:

A-i-BCSAB-l-C calculations of the total heat contents for variousreactions. Sources in chemical literature for the various heats forformation are given. The first group of reactions is between variousopaque materials and photoconductors and the second group is betweenvarious transparent conductive materials and photoconductors.

Group I.--Each reaction shown includes an opaque material and aphotoconductor on the left side of the equation and possible reactionproducts on the right side. Note that although more than one chemicalreaction is possible between each pair, if any reaction between eachpair is spontaneous then the photoconductor is degraded. Therefore, acomputation is made for only one of the possible reactions in each case.Values for heats of formation are in kilogram-calories per mole.

(A) Chromium (and chromium alloys) and antimony trisulfide:

3Cr-l-Sb S 3CrS-l-2Sb Heat of formation:

The right side is more negative, therefore reaction is spontaneous tothe right.

(B) Platinum and antimony trisulfide:

3Pt+Sb S S3PtS+2Sb Heat of formation:

Again, the right side is more negative, so reaction is spontaneous tothe right.

(C) Chromium (and chromium alloys) and selenium (and mixtures):

No thermodynamic data are available from literature for the feasibilityof reaction between chromium and selenium but the synthesis of thecompound chromium selenide from the elements is well known.

(D) Platinum and selenium (and mixtures):

No thermodynamic data are available from the literature but many workershave reported the reaction of platinum with selenium.

Group II.In the following equations a transparent conductive materialand a photoconductor are shown on the left side of the equation and aset of reaction products on the right side. Note that in order to insurethat the present invention will perform properly no chemical reactionmust take place. That is, any reaction that can be Written must have amore negative value for total heat content on the left side of theequation than on the right. Because of the number of possible reactions,it is not considered necessary to set forth all of these. One example ofreaction product for each combination of materials will suffice to showthe principle.

(A) Tin oxide and antimony trisulfide:

Heats of formation:

-144.0 4249.2 673.6 l67.4 112.ss

Iotal: -1s93.2 -953.ss

1 NBS Bulletin #500 (February 1952).

Hourly and Thomas, J. Chem. Soc. 86, 1417 (106-1).

3 Grnelin, v. 8, part B, 338 (1062 ed.).

' Mellor, A Comprehensive Treatise of Inorganic Chemistry.

side of the reaction is very small. However, laboratory experiments withsuch coatings in combination show that if any reaction takes place it isnegligible for the purposes of this invention.

(C) Tin oxide and selenium (and mixtures):

Heat of formation: 138.8 55.0

The left side is more negative than the right, therefore there is noreaction between tin oxide and selenium.

(D) Indium sesquioxide and selenium (and mixtures):

Thermodynamic data are not available from the literature on theselenides of indium. However, the heat of formation of In O is even morenegative than SnO Laboratory tests show no detectable reaction.

Thus, it is seen that the photoconductor is protected from opaquematerials reactive therewith by covering the opaque material with aconductive coating which is nonreactive.

Photoconductors which may be degraded by opaque materials includesantimony trisulfide, selenium, and mixtures containing selenium. Suchmixtures include, for example mixtures of selenium and sulfur.

It should be understood that the term optical as used herein refers tothe general region of the electromagnetic spectrum which includes theultraviolet region, invisible light region, and the infrared region.Furthermore, the term opaque is intended to mean substantiallynon-transparent with respect to the radiation which is to be viewed bythe device.

Although preferred embodiments of this invention have been shown anddescribed herein, the invention is not limited to such embodiments, butonly as set forth by the following claims.

I claim:

1. A photosensitive target comprising a transparent substrate,

an opaque material deposited in a pattern of a desired configuration onsaid substrate,

a transparent conductive coating covering said opaque material, and

a photoconductive layer on said conductive layer,

said opaque material being chemically or electrically reactive with thephotoconductive material,

and said conductive layer being chemically and electrically non-reactivewith the photoconductive material.

2. A photosensitive target as defined by claim 1 wherein the opaquematerial forms a dark current reference mask directly on the substrate.

3. A photosensitive target as defined by claim 1 wherein the opaquematerial is platinum, chromium, rhodium, palladium or an alloy of one ormore of them, the conductive coating is tin oxide or indium sesquioxide,and the photoconductive material is antimony trisulfide, selenium, ormixtures containing selenium.

4. A vidicon tube comprising an evacuated envelope,

at target in one end of said envelope, and

an electron gun in said envelope for producing an electron beam to scansaid target,

said target comprising a transparent substrate,

an opaque material deposited in a pattern of a desired configuration onsaid substrate on the side toward said electron gun,

a transparent conductive coating on said substrate covering said opaquematerial and the scanned area of said target, and

a photoconductive coating on said conductive coating,

said conductive coating being chemically and electrically non-reactivewith said photoconductive coating, and said opaque material being onewhich is chemically or electrically reactive with the material of saidphotoconductive coating.

1 NBS Bulletin #500 (February 1952).

5. A vidicon tube as defined by claim 4 wherein said opaque materialdefines a transparent window, and said electron beam scans an area ofsaid target which overlaps the edges of said window.

6. A photosensitive target comprising a transparent substrate,

an opaque material formed in a pattern on said substrate,

a layer of transparent conductive material selected from the groupconsisting of tin oxide and indium sesquioxide covering said opaquematerial, and

a photoconductive layer on said conductive layer,

said opaque material being one which is chemically or electricallyreactive with the photoconductive material.

7. A photosensitive target as defined by claim 6 wherein the opaquematerial is selected from the group consisting of chromium, platinum,rhodium, palladium and alloys of each.

8. A vidicon tube comprising an evacuated envelope,

a target in one end of said envelope, and

an electron gun in said envelope for producing an electron beam to scansaid target,

said target comprising a transparent substrate,

an opaque material deposited in a pattern of a desired configuration onsaid substrate on the side toward said electron gun,

a layer of transparent conductive material selected from the groupconsisting of tin oxide and indium sesquioxide on said substratecovering said opaque material and the scanned area of said target, and

a photoconductive coating on said conductive coating.

9. A vidicon tube as defined by claim 8 wherein the opaque material isselected from the group consisting of chromium, platinum, rhodium,palladium and alloys of each.

10. A vidicon tube as defined by claim 9 wherein the photoconductivecoating is selected from the group consisting of antimony trisulfide,selenium, and mixtures containing selenium.

References Cited UNITED STATES PATENTS 2,851,625 9/1958 Ruedy et al313-65 2,875,359 2/1959 Cope 31394 X 3,001,012 9/1961 Braicks 178-5.43,026,416 3/1962 Weimer 250-211 3,290,530 12/1966 Heagy 31365 3,310,7003/1967 Dresner et al 313-94 X OTHER REFERENCES RCA Technical Notes, RCATN No. 123, 1958; Gray,

Vidicon With Target Reticle.

ROBERT SEGAL, Primary Examiner.

U.S. Cl. X.R. 3l3109.5

